Liquid crystals are used primarily as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to the person skilled in the art and can be based on various effects. Such devices are, for example, cells having dynamic scattering. DAP cells (deformation of aligned phases). guest/host cells. TN cells having a twisted nematic structure. STN cells ("super-twisted nematic"). SBE cells ("super-birefringence effect") and OMI cells ("optical mode interference"). The most common indicating devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid crystal materials must have a good chemical and thermal stability and a good stability towards electric fields and electromagnetic radiation. Further, the liquid crystal materials should have a low viscosity and in the cells should give short response times, low threshold potentials and a high contrast. Furthermore, at usual operating temperatures, that is, in a range below and above room temperature which is as broad as possible, they should have a suitable mesophase, for example a nematic or cholesteric mesophase for the cells referred to above. In addition, there has also recently been an increased interest in the especially rapidly switching ferroelectric liquid crystals having a chiral tilted, smectic mesophase, especially a smectic C phase.
Since liquid crystals are generally used as mixtures of several components, it is important that the components have a good miscibility with one another. Other properties such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy must fulfil different requirements depending on the type of cell and field of application. For example, materials for cells having a twisted nematic structure should have a positive dielectric anisotropy and an electrical conductivity which is as small as possible.